Add facility for emitting heap profile marker

When analysing heap profile diagrams I often want correlate certain points in the program execution with the corresponding point in the heap profiling diagram.

Currently, I resort to causing flatlines by burning cpu cycles with some dummy calculations which don't cause too much GC activity. But this is just a crutch, and doesn't work too well for concurrent/parallel Haskell programs.

It would be helpful to have a GHC facility to emit custom marker "events" (maybe with an user-suppliable associated numeric identifier to differentiate between multiple markers) to the heap-profile which could then be directly looked up in the resulting heap profile diagram (rather than having to recognize geometric graph features in the diagram).

Trac metadata

Trac field	Value
Version
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Profiling
Test case
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